WO2000016431A1

WO2000016431A1 - Planar ortho-mode transducer

Info

Publication number: WO2000016431A1
Application number: PCT/US1999/020732
Authority: WO
Inventors: Nicolas Haller; Scott C. Cook
Original assignee: Channel Master Llc
Priority date: 1998-09-11
Filing date: 1999-09-08
Publication date: 2000-03-23
Also published as: US6087908A; AU5706499A

Abstract

An ortho-mode transducer has a common waveguide (12) and two orthogonal port waveguides (14, 16), each of which is also orthogonal to the common waveguide (12). Each port waveguide (14, 16) is coupled to the common waveguide (12) with a coupling aperture (26, 28) which will pass signals having a particular polarity and cut off orthogonally polarized signals. The common waveguide (12) is terminated in a shorting plane (24) about 1/4 of the signal wavelength from the vertical midpoint of the port waveguides (14, 16). The shorting plane (24) directs energy from the common waveguide (12) into the port waveguides (14, 16) and directs energy from the port waveguides into the common waveguide.

Description

PLANAR ORTHO-MODE TRANSDUCER

Technical Field

This invention is related to a waveguide device which supports two

orthogonal signal modes. More specifically, this invention is related to an ortho-

mode transducer in which the two orthogonal ports are realized in the same

plane.

Background Of The Invention

An ortho-mode transducer ("OMT") is a three-port waveguide device

which supports signals having two orthogonal modes. For purposes of

discussion, the two orthogonal signal modes will be designated as H and V linear

polarities. A conventional OMT is shown in Fig. 1 . The common port (port 1 )

is a circular, square, or similar type of waveguide portion which supports both

H and V polarization signals. The through port (port 2) is a waveguide portion

aligned with the common port waveguide and which supports only V polarized signals. Port 3, the side port, is a waveguide which splits off from the common

and through port waveguides and supports only H polarized signals.

OMTs are often used in reflector antenna systems to separate H and

V polarized signals. The combined signal is received, i.e., as focused energy

from a parabolic reflector, and applied to the common port of the OMT through

a feedhorn. The received V and H polarized signals are separated and output via

the through and side ports, respectively. OMTs are also used in applications

when the antenna system transmits H polarized signals and receives V polarized

signals. For this application, the H polarized output signal is transmitted from

a power amplifier module into the through port of the OMT, where it is directed

into the common port and output into the feed horn and the reflector. V

polarized signals are funneled by the feed horn into the common port of the

OMT, where it is directed into the side port and into a receiver module

(containing, for example, a filter, amplifier, down converter, etc.) . For receive

only antenna systems or transmit/receive antenna systems the orthogonal

through and side ports can be designed to cover the same, distinctly different or

overlapping frequency bands.

Good port to port isolation is critical to applications that transmit

from the V port and receive on the H port because the power transmitted from

the V port toward a distant satellite or terrestrial hub is very high in comparison

to the low power received at the H port. In conventional OMT designs, signal

separation and isolation between the through and side ports is achieved by

providing a septum or reduction in height in the body of the OMT near the

junction between the common and through waveguide portions. The septum or height reduction redirects H polarity signals from the common port into the side

port, while allowing V polarity signals from the common port to continue into the

through port. The arrangement also works in reverse, channeling both V polarity

signals entering the through port and H polarity signals entering the side port into

the common port. This mechanism, together with the orthogonal orientation of

the through and side ports, provides relatively good isolation between through

and side ports. In other words it allows only a small amount of the energy of H

polarity signals to enter the through port and very little V polarity signal energy

to enter the side port.

Although conventional OMT designs offer good port to port isolation

and functionality, the structure is asymmetric with respect to the common port

because the through, or V port is aligned with the common port, while the side,

or H port is orthogonal to the common port. This asymmetry can degrade port

to port isolation. It can also result in degraded cross polarity (x-pol) rejection,

i.e., the V port's rejection of the H polarity coming from the common port, and

the H port's rejection of V polarity coming from the common port.

Furthermore, because all three ports lie in the same plane, and

because the V port is axially aligned with the common port, the feed antenna

connected to the common port will lie along the same axis as any transmit or

receive elements connected to the V port. This results in a bulky assembly

which is unsuitable for many applications.

Accordingly, it is an object of the invention to provide an OMT

wherein both the H and V ports are in the same plane and are orthogonal to the

common port. It is a further object of the invention to provide an OMT with

improved cross polarity rejection.

Yet another object of the invention is to provide an OMT which may

be inexpensively fabricated as two planar elements joined together, which

elements contain the necessary filters, waveguides, etc. for integrating the OMT

and with a transmit package and/or a receive package.

Summary Of The Invention

According to the invention, a planar OMT is provided in which the

H and V ports both lie in a plane which is substantially orthogonal to the

common port. The common waveguide is terminated in an appropriately placed

short which forces the energy into the H and V ports, as opposed to the

conventional design which directs the H and V mode signals by using a reduced

height wave guide or a septum. If the frequency bands of the two polarities are

the same, the short is positioned approximately 1 /4 wavelength away from the

center of the H and V ports. If the frequency bands of the two polarities are

significantly different, one or more ridges may be placed in the end of the

shorting wall lined up with the higher frequency to provide more optimum

distance for matching.

According to a further aspect of the invention, the isolation and

cross polarity rejection between the H and V ports is increased by connecting the

H port to the common port with two sub-ports which enter the common port at

opposite sides and, preferably, substantially perpendicular to and in the same

plane as the V port. Because there is a 1 80° phase difference in the signals at the two sub-ports, the sub-ports are arranged so that distance between one sub-

port and the H port is 3 wavelength longer than the distance from the other sub-

port to the H port in order to properly combine them.

In yet a further aspect of the invention, the OMT is fabricated in

two pieces (top and bottom) which are fastened together along a plane common

to the H and V ports. All of the necessary filters, waveguides, and

transmit/receive microwave housing can be formed in these two OMT elements,

greatly reducing the number of housings and connections, which in turn reduces

cost and improves performance. In addition, because of the orthogonal

relationship between the ports, when H and V signals are received and extracted

by the new OMT, the output polarities of the signals are aligned, thus making it

easier to integrate the OMT with downstream elements. Similarly, when

transmitting orthogonal H and V signals, the two signals may initially be

presented to the OMT with the same polarity. The orthogonal relationship

between the ports will create a 90 degree difference in the polarity of the signals

as they are fed into the common port to provide orthogonally polarized signal

components.

Brief Description of the Drawings

The foregoing and other features of the present invention will be

more readily apparent from the following detailed description and drawings of

illustrative embodiments of the invention in which:

FIG. 1 shows a conventional OMT design; FIG. 2 is a perspective view of a planar OMT according to the

invention;

FIGs. 3a and 3b are side and top views, respectively, of the OMT

shown in Fig. 2;

FIG. 4 is a top view of a planar OMT according to a second aspect

of the invention; and

FIGs. 5a and 5b show a transceiver unit including the planar OMT

of Fig. 4 fabricated according to a further aspect of the invention.

Detailed Description of the Preferred Embodiments

Turning to Figs. 2, 3a, and 3b, there is shown an OMT 1 0

according to the present invention. OMT 1 0 has a common port 1 2, and two

side ports 14, 1 6. For purposes of discussion side port 1 4 will be referred to as

the "H port" and side port 1 6 will be referred to as the "V port." The use of H

and V is used here for simplicity and is not intended to limit the polarity of the

signals carried by the side ports 14, 1 6, or to limit the polarizations to only

orthogonally polarized signals.

Common port 1 2 is a waveguide, aligned along the common axis

C, which is suitable for carrying at least two differently polarized signals,

represented in Fig. 2 as polarity vectors 20, 22. Signal 20 has a first

polarization, designated "V", and is centered about frequency f(v) with

wavelength λ(v) . Signal 22 has a second polarization, designated "H", and is

centered about frequency f(h) with wavelength λ(h) . Although a circular

waveguide structure is shown, those of skill in the art will recognize that other configurations, such as rectangular or oval, may also be used, particularly if the

frequency bands of the two polarities of signals to be carried are not the same,

i.e., f(v) and f(h) are different or the expected bandwidth of the V and H signals

20, 22 is not the same.

The H port 1 4 is a waveguide structure, here shown as a

rectangular waveguide, which is coupled to the common port 1 2 by a suitable

coupling aperture 26. Port 1 4 is aligned along the H axis. Aperture 26 is

configured to pass signals of a given polarity, such as a signal 22, when the

OMT 1 0 is properly aligned with the plane of polarization of the signal. In the

embodiment shown in Fig. 2, the H axis is perpendicular to plane of polarization

for the H signal 22. The plane of polarization may represent either the magnetic

or electric field, depending on the type of coupling aperture utilized. Designs for

coupling apertures of this type are well known to those skilled in the art.

Waveguide port 14 is configured to carry such a polarized signal.

The V port 1 6 is a waveguide structure, here shown as a

rectangular waveguide, which is coupled to the common port 1 2 by a suitable

coupling aperture 28. Port 1 6 is aligned along the V axis and coupled to the

common port 1 2 at a predetermined angle relative to the H axis. The specific

angle is determined by the relative difference in polarity orientation between the

two signal components 20, 22. When the OMT 10 is properly aligned, the V

axis is perpendicular to the plane of polarization for the V signal 20.

In the preferred embodiment, the two signal components 20, 22 are

orthogonally polarized signals and port 1 6 is coupled to the common port 1 2 at

substantially a 90 degree angle relative to port 1 4, such as shown in the figures. Aperture 28 is configured to pass signals of a given polarity, such as signal 20,

which is horizontally polarized, and the waveguide of port 1 6 is configured to

carry such a polarized signal.

The common port 1 2 terminates in a short 24, such as a conducting

wall, which forces energy carried by the common port 1 2 into the H and V ports

14, 1 6. To achieve this result, the short 24 is positioned approximately an odd

number of quarter wavelengths from the vertical mid-point or center 30 of the

V and H ports 1 4, 1 6 (when the frequency of the H and V components are

substantially the same). In other words, the short position is approximately a

distance of λ* (2n + 1 )/4 from the vertical mid-point, where n is an integer greater

than or equal to zero. In the preferred embodiment, the short is positioned

approximately ¹Λ wavelength from the center 30 to maximize the usable

bandwidth of the device.

If the frequency bands of the two polarity signals 20, 22 are

significantly different. The shorting wall 24 is preferably positioned ¹Λ

wavelength from the center of the side port which will carry the lower frequency

and longer wavelength signal. For example, if f(v) is significantly lower than f(h),

the short 24 is placed approximately ¹Λ λ(v) from the center of V port 1 6. To

provide an appropriate shorting point for the higher frequency side port, here H

port 1 4, one or more ridges 32 which are lined up with the higher frequency

polarity port 1 4 can be placed in the common port 1 2 to provide a short which

is visible only for the H polarity signal. The appropriate dimensions and number

of ridges to achieve a "virtual" shorting point at ¹Λ λ(h) from the center of H port

1 4 depend on the geometry and operating frequency of the OMT 1 0 and techniques for selecting the appropriate waveguide impedance divider

characteristics are known to those of skill in the art.

The OMT 10 may be used to separate two orthogonally polarized

input signals 20, 22 having V and H polarization. Signals 20, 22 are received,

i.e., through a horn feed, and channeled into the common port 1 2. The signal

components are reflected by the terminating short and directed towards the sides

of the common port waveguide 22. Different polarity signal components may

be extracted by connecting the side ports to the common port 1 2 at

appropriately positioned aperture locations.

For example, as illustratively shown with the V and H signal vectors

of Fig. 2, the relative polarity of the signal components as they are directed

outwards from the axis of the common port and into the side ports 14, 1 6 is

dependent on the position along the axis at which the signal is measured. As

shown, the coupling aperture 26 is configured such that the V polarity signal 20

is cut off and therefore does not see the H port 14. The coupling aperture 28

is aligned such that it accepts V polarity signals 20. Further, the V port 1 6 is

configured to accept the V polarity signal 20 and pass it through to components

downstream from the V port 1 6. Similarly, the coupling aperture 28 is

configured to cut off the horizontal signal component 22, whereas the aperture

26 accepts and passes the H polarity signal 22 to the horizontal port 1 4.

Although the OMT 10 has been discussed with respect to receiving

differently polarized signals, the device may also be used in reverse. Signals

having aligned polarities which are input to the H and V ports 1 4, 1 6 are

transmitted through the OMT 10 to provide orthogonal signal components which output from the common port 1 2. OMT 10 may also be used as part of a

transducer, where, for example, V polarity signals are received and H polarity

signals are transmitted.

The OMT 1 0 illustrated in the figures is an H-plane OMT in that the

ports and 14, 1 6 and apertures 26, 28 have their longer wall parallel to the

common waveguide 1 2 (i.e., the ports are tall and skinny). However, OMT 10

may also be formed in an E-plane configuration, where the long wall is

perpendicular to the common mode waveguide 1 2 (i.e., the ports are short and

wide) . Other configurations may also be used, provided that the apertures admit

the proper polarity signals and the ports carry those signals.

According to a further aspect of the invention, shown in Fig. 4, the

cross polarity rejection of the OMT 1 0' is improved by increasing the symmetry

of at least one of the side ports 14, 1 6. This is accomplished by replacing a

single port 1 6 with two sub-ports 1 6a and 1 6b, which are coupled to the

common mode waveguide 1 2 at opposing points substantially 1 80 degrees from

each other. The coupling is achieved through suitably configured coupling

apertures which pass signals having the desired polarization, here the V

polarization signal 20, as discussed above.

These two ports (1 6a and 1 6b) are in the same plane and are

combined in the same plane with intermediate waveguides 34 and 36 coupled

to single port 1 6 by a waveguide impedance divider 37. As illustrated, signals

entering waveguides 34, 37 from waveguide 1 6 at the impedance divider are

1 80 degrees out of phase. To account for this phase difference, the length of

waveguide 36 from port 1 6b to the divider 37 is an odd number of one half wavelengths longer, preferably 1 /2λ(v), than the length of waveguide 34 from

port 1 6a to the divider 37. Preferably, waveguides 34 and 36 are rectangular

and have a length differential which is half the center frequency of the signal

component processed by the respective port 1 6, i.e., λ(v)/2.

According to a further aspect of the invention, shown in Fig. 5a, the

OMT 1 0' (or 10) may be constructed of two generally planar pieces or blocks

40, 42 (top and bottom) that can be fabricated using conventional techniques,

such as machining, casting, or both, and then fastened or otherwise assembled

together. The two pieces each contain upper and lower portions of the OMT

structure components. For example, with reference to Fig. 2, the OMT may be

divided into two parts separated along the plane defined by or at least parallel to

the H and V axes. A portion of the common and port waveguides is formed into

each block 40, 42. All of the necessary filters, waveguides, and transmit/receive

microwave housing can be built (machined or cast) into these same two pieces

40, 42. This greatly reduces the number of housings and connections, which

in turn reduces cost and improves performance. Figure 5a illustrates a unit 38

which integrates the OMT 1 0', filters 44, and a transmitter or receiver package

48 into a single package. Also provided is an output port 50 to which a second

transmitter or receiver package may be connected. Alternatively, the pieces 40,

42 forming unit 38 may be extended to integrate the second transmitter or

receiver in a manner similar to the first 48, to thereby form an integral

transceiver unit fabricated from a minimum number of parts.

Fig. 5b is an exploded view of the unit 38, further including a feed

horn 54 which attaches to the common port 1 2 of the OMT 1 0' via a suitable coupler 52. Because the feed horn 54 is perpendicular to the rest of the

transceiver structure, a very compact assembly may be produced.

While the invention has been particularly shown and described with

reference to preferred embodiments thereof, it will be understood by those skilled

in the art that various changes in form and details may be made therein without

departing from the spirit and scope of the invention.

Claims

WE CLAIM:

1 . An ortho-mode transducer comprising:

a common waveguide aligned along a central common axis and

configured to carry a first polarized signal having a first wavelength and a second

polarized signal having a second wavelength, said first and second signals having

polarity vectors differing by a predetermined angle, said common waveguide

having a shorting point;

a first port waveguide configured to carry said first signal, said first

port waveguide being coupled to said common waveguide above said shorting

point with a first coupling aperture and being aligned along a central first port

axis which is substantially perpendicular to the central common axis, said first

coupling aperture configured to pass said first signal and cut off said second

signal when said first port axis is perpendicular to the plane of polarization of

said first signal;

a second port waveguide configured to carry said second signal,

said second port waveguide being coupled to said common waveguide above

said shorting point with a second coupling aperture and being aligned along a

central second port axis which is substantially perpendicular to the central

common axis and offset from said first port axis by substantially said

predetermined angle, said second coupling aperture configured to pass said

second signal and cut off said first signal when said second port axis is

perpendicular to the plane of polarization of said second signal; and said shorting point being approximately an odd number of quarters

of said first wavelength from the vertical midpoint of said first port waveguide

at said first aperture.

2. The transducer of claim 1 , wherein:

said first and second wavelengths are substantially the same;

said shorting point being approximately an odd number of quarters

of said second wavelength from the vertical midpoint of said second port

waveguide at said second aperture.

3. The transducer of claim 1 , wherein said first wavelength is

substantially longer than said second wavelength, said common waveguide

further comprising one or more ridges at said shorting point and aligned with said

second port, said ridges configured to provide a virtual shorting point for said

second signal at approximately an odd number of quarters of said second

wavelength from the vertical midpoint of said second port waveguide at said

second aperture.

4. The transducer of claim 1 , wherein said predetermined angle

is substantially 90 degrees.

5. The transducer of claim 1 , further comprising: a third port waveguide configured to carry said second signal and

coupled to said common waveguide above said shorting point with a third

coupling aperture;

said third port waveguide opposing said second port waveguide and

being centrally aligned along said second port axis, said third coupling aperture

configured to pass said second signal and cut off said first signal when said

second port axis is perpendicular to the plane of polarization of said second

signal; and

an impedance divider connected to said second port waveguide by

a first intermediate waveguide having a first length and to said third port

waveguide by a second intermediate waveguide having a second length, said

first and second lengths differing by substantially an odd number of halves of

said second wavelength.

6. The ortho-mode transducer of claim 1 wherein:

said common waveguide, first port waveguide and second port

waveguide each being comprised of a respective upper and lower portion;

a first block having the upper portions of said common waveguide,

first port waveguide, and second port waveguide formed therein;

a second block having the lower portions of said common

waveguide, first port waveguide, and second port waveguide formed therein;

said first block adjacent said second block and positioned such that

said upper portions are substantially aligned with said lower portions.

7. A transducer comprising:

a first and second generally planar blocks;

a common waveguide having an upper portion formed in said first

block and a lower portion formed in said second block;

said common waveguide aligned along a central common axis

substantially perpendicular to said first and second blocks and configured to

carry a first polarized signal having a first wavelength and a second polarized

signal having a second wavelength, said first and second signals having polarity

vectors differing by a predetermined angle, said common waveguide having a

shorting point formed in said second block;

a first port waveguide having an upper portion formed in said first

block and a lower portion formed in said second block;

said first port waveguide configured to carry said first signal and

being coupled to said common waveguide above said shorting point at a first

coupling aperture and aligned along a central first port axis which is substantially

perpendicular to the central common axis;

said first coupling aperture having an upper portion formed in said

first block and a lower portion formed in said second block and configured to

pass said first signal and cut off said second signal when said first port axis is

perpendicular to the plane of polarization of said first signal;

a second port waveguide having an upper portion formed in said

first block and a lower portion formed in said second block;

said second port waveguide configured to carry said second signal

and being coupled to said common waveguide above said shorting point with a second coupling aperture and being aligned along a central second port axis

which is substantially perpendicular to the central common axis and offset from

said first port axis by substantially said predetermined angle;

said second coupling aperture having an upper portion formed in

said first block and a lower portion formed in said second block and configured

to pass said second signal and cut off said first signal when said second port

axis is perpendicular to the plane of polarization of said second signal; and

said shorting point being approximately an odd number of quarters

at said first aperture.

8. The transducer of claim 7, further comprising:

a third port waveguide having an upper portion formed in said first

block and a lower portion formed in said second block;

said third port waveguide configured to carry said second signal and

coupled to said common waveguide above said shorting point with a third

coupling aperture, said third port waveguide opposing said second port

waveguide and being centrally aligned along said second port axis;

said third coupling aperture having an upper portion formed in said

first block and a lower portion formed in said second block and configured to

pass said second signal and cut off said first signal when said second port axis

is perpendicular to the plane of polarization of said second signal; and

an impedance divider connected to said second port waveguide by

a first intermediate waveguide having a first length and to said third port waveguide by a second intermediate waveguide having a second length, said

first and second lengths differing by substantially an odd number of halves of

said second wavelength.

9. The transducer of claim 7, further comprising at least one of

a filter section, receiver section, and transmitter section formed in said first and

second blocks and being in electrical communication with said first waveguide

port.

10. The transducer of claim 7, wherein said upper portion of said

common waveguide extends through said first block and terminates in a coupler

suitable for receiving a feed horn.